The present invention pertains to a method for strengthening a structural steel channel member and, more particularly, to a method of forming strengthening lips on the end flanges of a channel member after the member has been heat treated.
High strength, low carbon steel channel members, sometimes referred to as C-channels, are widely used as the longitudinal side rails in the fabrication of truck frames. A typical C-channel includes a central web and a pair of parallel flanges extending perpendicularly from the opposite edges of the web. C-channels are typically cold rolled from a low carbon steel and then initially heated to develop a desired austenite grain structure which is converted to a martensite structure by rapid quenching in water, and then tempered to create a desired toughness. The foregoing process is particularly desirable for C-channel members used as side rails in heavy truck frames where steel having a tensile strength well in excess of 50,000 psi is required.
The rapid, high volume water quench used to convert the austenite grain structure to martensite is known to cause extreme distortion of the C-channel member. Such distortions may be removed after quenching, but the preferable method has been to utilize quenching dies that restrain the member from distortion while a high volume flow of water is directed through the die to all surfaces of the member. However, quenching dies are extremely costly and are only practical for use in very high volume standard steel sections.
It is also known that the stiffness and strength of a C-channel can be increased by forming a small lip on the free edge of each of the flanges. Such lips are formed by rolling or otherwise turning the edges of the flanges toward one another such that the lips extend generally perpendicular to the flanges. Although it would be possible to form strengthening lips on a C-channel in the initial cold rolling process from which the member is formed, such preformed sections would require even more complex and costly quenching tooling than a C-channel without preformed strengthening lips. This is because a typical quenching die utilizes a collapsible configuration that necessarily becomes even more complex when it must be constructed to accommodate the presence of in turned lips. Furthermore, because heavy truck manufacturers have varying size and gauge requirements for C-channels used as frame side rails, customized quenching dies would be required for each different size and gauge, a situation that would be completely cost-prohibitive.
In accordance with the present invention, a method is provided for forming strengthening lips on flanges of a channel member after heat treating. The preliminary heat treating comprises the steps of heating the member to an austenitizing temperature of at least about 1400xc2x0 F.; quenching the member in a quenching die; and reheating the member to a tempering temperature of at least about 800xc2x0 F., followed by the step of forming lips along the edges of the flanges while the member is still hot, preferably at or close to the tempering temperature.
The method of the present invention is particularly well suited for channel sections made from low carbon steels (having a carbon content in the range of about 0.20 to 0.30 weight percent). Such steels are amenable to heat treating as described above to tensile strengths in excess of 100,000 psi.
The preliminary austenitizing step is preferably performed at a temperature in the range of about 1400-1700xc2x0 F. After quenching, the tempering step is preferably performed at a temperature in the range of about 800-1000xc2x0 F. The final lip forming step is preferably performed at a temperature in the range of about 500-900xc2x0 F., more preferably in the range of about 800-900xc2x0 F.
The strengthening lips are preferably formed by rolling. The rolling step is preferably performed with a series of progressive rollers. The formed strengthening lips may extend from the flanges inwardly at an angle of about 90xc2x0, but an angle in the range of about 80xc2x0 to 100xc2x0 is satisfactory.